Horizontal scanning rate correction apparatus

ABSTRACT

A horizontal scanning rate correction apparatus for a cathode-ray tube, particularly of the beam index color cathode-ray tube type, includes a memory for storing correction values representing deviations of the horizontal scanning rate of the electron beam from a desired scanning rate at each of a plurality of horizontal sampling positions along each of a plurality of predetermined horizontal sampling lines which are substantially fewer in number than the horizontal lines along which scanning occurs. During display of a video signal, a selected one of the stored correction values is read out of the memory for each scanning position of the beam along a scanned one of the horizontal lines, with each read correction value, and a corresponding sampling line correction signal produced therefrom, representing the deviation at a corresponding horizontal sampling position on one of the sampling lines. A scanned line correction signal is then produced for each horizontal scanning position which is a function of the sampling line correction signal and of the vertical position of the respective scanned line, and the scanned line correction signal is applied to the beam deflection yoke or device to substantially cancel the scanning rate deviation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to apparatus for controlling the horizontalscanning rate in cathode-ray tubes, and more particularly is directed tosuch an apparatus which is adapted for use with a beam index colorcathode ray tube.

2. Description of the Prior Art

In cathode-ray tubes in which an electron beam is caused to repeatedlyscan across the screen in a vertical succession of horizontal lines, itis important to control the rate at which the electron beam travelsacross each horizontal line. It is common for the picture informationwithin a video signal to be timed in such a manner that it will beprojected with the proper shape upon the screen of a cathode-ray tubeonly if the electron beam travels across each fraction of a horizontalline at a specified rate. Deviations of the horizontal scanning ratefrom such a specified rate cause distortions in the shape of theprojected image and are thus undesirable.

Deviations in the horizontal scanning rate are particularly undesirablein beam index color television receivers because such deviations mayalso cause color misregistration in such receivers.

Beam index color television receivers are well known in the prior art,and usually include a cathode-ray tube, or picture tube having anelectron gun which emits a single electron beam and a phosphor screenhaving a repeating pattern of red, green and blue primary color phosphorstripes extending vertically upon the screen. The beam index picturetube also has a plurality of vertical index phosphor stripes spacedacross its phosphor screen in a known relationship to the spacing of thecolor phosphor stripes. When the electron beam horizontally scans thescreen, a photodetector generates an index signal in response to thelight emitted each time an index stripe is struck by the electron beam.This index signal is used to achieve color registration by controllingthe color switching circuit which determines when the three primarycolor signals respectively modulate the intensity of the electron beamso that, at any moment, the intensity of the electron beam is modulatedby the primary color signal whose corresponding color phosphor stripe isthen being scanned by the beam.

In such beam index color television receivers deviations in thehorizontal scanning rate cause color misregistation because there is aninherent delay in the response of the color switching circuit to changesin the horizontal scanning rate of the beam and it is difficult tocompensate therefor. This delay results from the fact that it is commonfor index signal processing circuitry, for example, comprised of abandpass filter and a phase-locked loop (PLL) circuit, to be interposedbetween a photodetector which detects the index signal and the colorswitching circuit. The bandpass filter removes unwanted noise from theindex signal in preparation for the application of that signal to theinput of the PLL circuit which provides an input to the color switchingcircuit of greater uniformity, in amplitude and frequency, than theindex signal. In addition, by insertion of a frequency divider in thefeedback loop of the PLL circuit, the latter can be made to produce anoutput frequency which is a predetermined multiple of the frequency ofthe index signal. The last feature is important since in most beam indexcolor cathode ray tubes the number of color phosphor stripes is notequal to, but instead is an integral multiple of, the number of indexstripes.

Unfortunately, the delay inherently associated with the above-describedindex signal processing circuitry, particularly with the PLL circuit,varies as a function of the frequency of the index signal, which in turnvaries in proportion to the horizontal scanning rate. For this reason, avariation in the scanning rate is not immediately or precisely reflectedin a corresponding change in the rate of color switching so thatdeviations in the horizontal scanning rate adversely affect colorregistration.

Deviations in the horizontal scanning rate are also disadvantageous inbeam index color television receivers because they make it moredifficult for the PLL circuit to correctly follow and lock onto theinstantaneous frequency of the index signal, as is necessary for propercolor registration. In order to cause the PLL circuit to properly followthe frequency of an index signal when the horizontal scanning ratedeviates, it is necessary to increase the minimal signal strength of theindex signal. This requires that the minimal intensity of the electronbeam be increased, which, in turn, has the undesirable result ofincreasing the luminance of the darkest areas that can be projected onthe picture tube, and, thus, of decreasing the contrast of the producedimage.

For all of the above reasons, it is desirable to limit the maximumfluctuations in the horizontal scanning rate of beam index colortelevision receivers to less than several tenths of one percent. Theprior art contains various proposed schemes for correction of thehorizontal scanning rate, but unfortunately none of them has been ableto limit the scanning rate fluctuation to the desired level of less thanseveral tenths of one percent.

In the copending application Ser. No. 99,911, filed Dec. 3, 1979, andassigned to the same assignee as this application, there is disclosed ahorizontal scanning rate correction apparatus having a memory forstoring corrections values representing deviations of the horizontalscanning rate from a desired scanning rate at a plurality of respectivehorizontal sampling positions along a plurality of horizontal lines, andcircuitry for reading the correction values from the memory as a videosignal is being displayed and for producing corresponding signalssupplied to a horizontal deflection device to substantially cancel theunwanted deviations in the horizontal scanning rate. The apparatusdisclosed in such copending application is designed for use with anindex beam color cathode-ray tube in which a PLL circuit including avoltage-controlled oscillator receives the index signal as an input andproduces an output having a frequency equal to the frequency at whichcolor phosphors are being scanned. In such a PLL circuit the controlvoltage fed to the voltage-controlled oscillator is proportional to thefrequency of the index signal, and it is from this control voltage thatthe correction values stored in the memory circuitry are derived.

The foregoing scanning rate correction apparatus can accurately canceldeviations occurring in a particular horizontal scanning line so long asthe correction values accurately correspond to the deviation of thehorizontal scanning rate along that line and as long as enoughcorrection values are stored in the memory to accurately indicatevariations in the scanning rate which occur along its length. Forexample, it has been found that, if accurate correction values arerecorded for 32 sampling positions along a given horzontal line, theapparatus will be able to cancel deviations in the horizontal rate alongthe line sufficiently to substantially remove the above-mentionedundesirable effects of deviations in the horizontal scanning rate.

Such horizontal scanning rate correction apparatus would produce idealresults if correction values were stored for each horizontal line of thevideo field. Unfortunately, this would require a very large andexpensive memory capacity, since there are 236 lines in the effectivepicture area of the video field of an NTSC video signal. For example, if32 8-bit correction values were stored for each of 236 lines, a memorycapacity of 60,416 bits would be required.

It would be possible to reduce the memory capacity required of such ahorizontal scanning rate correction apparatus by storing correctionvalues for less than the total number of horizontal lines in theeffective picture portion of each field, for example, for every 16thhorizontal line, and then substituting correction values from a nearbyline for which values have been stored when scanning a horizontal linefor which correction values have not been stored. Unfortunately, such amethod of correcting for deviations in the horizontal scanning rateoften produces poor results. If a group of horizontal lines at variousvertical positions all have their scanning rate corrected according tocorrection values stored for only one of such lines, no compensation isprovided for the difference in the horizontal scanning rates occurringas a result of the different vertical positions. Furthermore, arelatively large difference will exist between the scanning ratecorrections made to a first group of horizontal lines correctedaccording to a set of correction values specific for one of those linesand that made to a next group of horizontal lines corrected according toa set of correction values specific for a line in such next group. Suchlarge difference between the scanning rate corrections is likely tocreate a distorted picture in which the groups of horizontal linescorrected according to different correction values appear as horizontalbands.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to providehorizontal scanning rate correction apparatus for cathode-ray tubes thatavoids the above-described defects inherent in the prior art.

Another object of the invention is to provide horizontal scanning rateconnection apparatus which substantially eliminates deviations in thehorizontal scanning rate of a cathode-ray tube by reading correctionvalues from a memory and using those values to generate correctionsignals supplied to a horizontal deflection device, and which canoperate effectively with a reduced memory capacity to accuratelycompensate for deviations in the horizontal scanning rate and therebyproduce a substantially distortion-free image.

Still another object of this invention is to provide a horizontalscanning rate correction apparatus, as aforesaid, which is particularlysuited for use with beam index color cathode-ray tubes of the typehaving a screen scanned by an electron beam, a plurality of indexelements positioned to be struck by the electron beam as it scans acrossthe screen, a beam deflection device supplied with at least horizontaland vertical beam deflection signals for causing the electron beam torepeatedly scan across the screen in a vertical succession of horizontallines, and an index signal processing circuit for producing an indexsignal of a frequency determined by the frequency of the incidence ofthe electron beam upon the index elements as it scans across thehorizontal lines and for controlling color switching circuitry whichdetermines which of a plurality of color signals modulates the intensityor density of the electron beam, with such processing circuit includinga phase-locked loop having a voltage-controlled oscillator, and with thecorrection values stored in the memory being derived from the controlvoltage for such oscillator.

In accordance with an aspect of this invention, a horizontal scanningrate correction apparatus for a cathode-ray tube having a screen whichis scanned by an electron beam in a vertical succession of horizontallines, comprises memory means for storing correction values representingdeviations of the horizontal scanning rate of the electron beam from adesired scanning rate at a plurality of predetermined horizontalsampling positions along each of a plurality of predetermined samplinglines from among the horizontal lines scanned across the screen by theelectron beam, reading means for reading a selected one of the storedcorrection values for each horizontal scanning position of the electronbeam along a scanned one of the horizontal lines, with each correctionvalue that is read representing the deviation of the horizontal scanningrate at a corresponding one of the horizontal sampling positions on oneof the sampling lines for which correction values have been stored, asampling line correction signal being produced in correspondence to eachof the correction values that is read from the memory means, means forproducing, for each horizontal scanning position, a scanned linecorrection signal which is a function of the sampling line correctionsignal then being produced and of the vertical position of that scannedhorizontal line, and means for supplying the scanned line correctionsignal to the beam deflection device or yoke of the cathode-ray tube sothat the deviation of the horizontal scanning rate along the scannedline is substantially cancelled.

According to another aspect of the invention, the scanned linecorrection signal is produced for scanning positions on scannedhorizontal lines other than the sampling lines for which correctionvalues have been stored by interpolating between two sets of samplingline correction signals produced by the reading means. Thisinterpolation is based on the vertical position of the scanned linerelative to the vertical positions of the sampling lines to which thecorrection values actually read from the memory means correspond.

According to yet another aspect of the invention, the beam deflectiondevice used with the cathode-ray tube includes the usual horizontal andvertical deflection coils for receiving the horizontal and vertical beamdeflection signals, respectively, and a separate correction coil placedon a yoke separate from the horizontal and vertical deflection coils forreceiving the deflection correction signal.

The above, and other objects, features, and advantages of the invention,will be apparent in the following detailed description of illustrativeembodiments of the invention which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a horizontal scanning rate correctionapparatus according to one embodiment of the present invention, andwhich is shown applied to a beam index color cathode-ray tube;

FIG. 2 is an enlarged fragmentary sectional view showing a section ofthe screen of the cathode-ray tube of FIG. 1;

FIGS. 3 and 4 are diagrams to which reference is made in explaining thewriting and reading, respectively, of information in a memory includedin the apparatus of FIG. 1;

FIG. 5 shows a correction coil included in the deflection device thecathode-ray tube in FIG. 1;

FIGS. 6 and 7 are waveform diagrams to which reference is made inexplaining the timing of the operation of the embodiment of theinvention shown in FIG. 1; and

FIG. 8 shows waveform diagrams representing various signals produced inthe embodiment of the invention shown in FIG. 1, and to which referencewill be made in explaining the operation thereof.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring first to FIG. 1, it will be seen that a beam index colorcathode-ray or picture tube 10 to which this invention may be applied isprovided with an electron gun 11 that emits a single electron beam, theintensity or density of which is modulated by a signal applied to a gridelectrode 12 forming part of electron gun 11. Picture tube 10 alsoincludes a phosphor screen 13 composed of a repeating pattern of red,green and blue primary color phosphor stripes R, G, B (FIG. 2) extendingvertically upon the inner surface of the glass face-plate or panel 14 ofpicture tube 10. A thin metallic layer 15, for example, a vacuumdeposited aluminum, covers the inner surface of screen 13 and istransparent to electrons of the electron beam while being effective toreflect toward the viewer the light emitted by the color phosphorstripes. A plurality of vertical index phosphor stripes I are spacedacross layer 15 on the inside surface of phosphor screen 13 in a knownrelationship to the spacing of color phosphor stripes R, G and B. A beamdeflection device 16 (FIG. 1) is supplied with horizontal and verticalbeam deflection signals for causing the electron beam emitted byelectron gun 11 to repeatedly scan across screen 13 in a verticalsuccession of horizontal lines.

An index signal processing circuit is associated with tube 10 and iscomprised of a photodetector 20, a bandpass filter 21 and a phase-lockedloop (PLL) circuit 22. PLL circuit 22 includes a phase-comparator 23, alow-pass filter 24, a voltage-controlled oscillator 25, and a frequencydivider 26. The index signal processing circuit is used for controllingcolor switching circuitry comprised of a mode set pulse generator 30, agate pulse generator 31, and a gate circuit 32, which determines wheneach of a plurality of color signals E_(R), E_(G) or E_(B) is suppliedto grid 12 so as to modulate the intensity or density of the electronbeam.

When the electron beam emitted by electron gun 11 horizontally scansscreen 13, photodetector 20 provided at the funnel-shaped portion ofpicture tube 10 generates an index signal in response to the lightemitted each time that an index stripe I is struck by the electron beam.The output signal from photodetector 20 is applied to bandpass filter 21for the purpose of removing therefrom certain signal components, suchas, those generated during the flyback period, which have frequenciesdifferent than the frequency with which the electron beam strikes indexstripes I during horizontal scanning intervals. The resulting indexsignal produced at the output of bandpass filter 21 has a frequencydetermined by the distance between index phosphor stripes I and thescanning speed of the electron beam. The index stripes I are uniformlyspaced apart across the image area of screen 13 so that the frequency ofthe index signal varies in proportion to the horizontal scanning rate ofthe beam during the horizontal scanning intervals. The index signal fromthe output of bandpass filter 21 is applied to one input of phasecomparator 23 in PLL circuit 22. The variable output voltage of phasecomparator 23 is applied, through low-pass filter 24 which removesunwanted noise therefrom, to the control input of voltage-controlledoscillator 25 which has a central frequency N times the normal frequencyof the index signal. The output from voltage-controlled oscillator 25 issupplied to the input of frequency divider 26 which divides thefrequency thereof by N, where N is an integral value representing thenumber of color phosphor stripes R, G, B between adjacent index phosphorstripes I. The output of frequency divider 26 is supplied to a secondinput of phase comparator 23 for phase comparison with the index signalderived from the output of bandpass filter 21.

As a result of the foregoing arrangement of PLL circuit 22, the outputof the voltage-controlled oscillator 25 will vary in frequency until thetwo input signals supplied to phase comparator 23 are of the samefrequency and phase. Thus, the output of voltage-controlled oscillator25 will have a frequency which is N times as great as the frequency ofthe index signal and three times as great as the so-called tripletfrequency at which the repeating patterns of red, green and blue colorphosphor stripes R, G, B are scanned by the electron beam, so that onepulse is generated by voltage-controlled oscillator 25 for each of thecolor phosphor stripes being scanned.

Each time the frequency of the index signal supplied to one input ofphase comparator 23 is varied, phase comparator 23 generates an outputvoltage which, when applied through low-pass filter 24, causesvoltage-controlled oscillator 25 to suitably vary its output frequencyand phase for restoring the equilibrium at comparator 23. Thus, it canbe seen that the voltage supplied by phase comparator 23 throughlow-pass filter 24 to the input of voltage-controlled oscillator 25varies with changes in the frequency of the index signal.

The output of PLL circuit 22 is applied to gate pulse generator 31,which, for example, may include a ring counter (not shown). Gate pulsegenerator 31 responds to each output pulse from voltage-controlledoscillator 25 by producing either a red, a green, or a blue gatingpulse, in response of the count of its ring counter. The resultingrepeated sequences of red, green and blue gating pulses are 120° out ofphase from each other and are supplied to respective control inputs ofgate circuit 32. In response to these repeated sequences of gatingpulses, suitable gates (not shown) in gate circuit 32 sequentiallyselect red, green and blue primary color signals E_(R), E_(G) and E_(B),respectively, and supply the gated or selected color signal through an Rcontact of a mode selector switch 33a to the input of a drive circuit 34which, in turn, provides the selected color signal to grid 12 so that itcan modulate the intensity or density of the electron beam projectedupon screen 13.

The index signal from bandpass filter 21 is also applied to mode setpulse generator 30 which may be of conventional design and is used toset or determine the phase relationship between the red, green and bluegating pulses from generator 31, and the scanning of the three primarycolor phosphors R, G, B. The mode set pulse generator 30 is requiredparticularly where the phase relationship between the index signal andthe color phosphor stripes R, G, B is not constant, for example, in beamindex cathode-ray tubes in which the index stripes I are separated byonly two color phosphor stripes, as is shown in FIG. 2, rather than by afull set of all three color phosphor stripes. The mode set pulsegenerator 30 may, for example, count a predetermined number of indexpulses arising from scanning of index stripes on a run-in area of thescreen to produce a mode set pulse which is applied to gate pulsegenerator 31. The mode set pulse causes the ring counter within gatepulse generator 31 to be set at the commencement of scanning of theimage area so that the gating pulses thereafter generated are in phasewith the colors of the phosphors then being scanned.

There are inevitable delays between the time that a particular indexstripe I is struck by the electron beam and the resulting gating pulseissue from generator 31 for controlling color switching by gate circuit32. For example, there are time delays introduced by the operation ofbandpass filter 21 and PLL circuit 22. In order to maintain proper colorregistration of the image produced upon screen 13 it is necessary thatsuch delays be compensated for so that the operation of gate circuit 32can be accurately synchronized with the actual scanning position of theelectron beam, and so that primary color signals E_(R), E_(G) and E_(B)

modulate the electron beam as that beam scans the corresponding colorphosphor stripes R, G and B, respectively. If such time delays areconstant they can easily be compensated for by properly choosing thetiming parameters of the components in the feedback loop constituted byphotodetector 20, bandpass filter 21, PLL circuit 22, gate pulsegenerator 31, gate circuit 32, drive circuit 34 and picture tube 10.Unfortunately, the delay associated with that feedback loop,particularly the delay associated with the phase difference between thetwo inputs of phase comparator 23, varies as a function of the frequencyof the index signal. For this reason, deviations in the horizontalscanning rate of the electron beam upon screen 13 make it difficult tomaintain proper color registration.

In order to prevent such deviations in the horizontal scanning rate,correction values that represent deviations of the horizontal scanningrate from a desired scanning rate at a plurality of horizontal samplingpositions along a plurality of predetermined horizontal scanning linesmay be stored in a memory. Such correction values can then be read fromthe memory to provide corresponding correction signals to beamdeflection device 16 by which deviations of the horizontal scanning rateare substantially cancelled.

The above-described apparatus can accurately cancel scanning ratedeviations occurring in a horizontal scanning line only so long as thecorrection values read from the memory during the reproduction of thatline accurately correspond to the then occurring uncorrected horizontalscanning rate of the electron beam. For example, enough information willbe provided to accurately compensate for deviations in the horizontalscanning rate for a given horizontal line if the control voltage E_(CV),shown graphically at the bottom of FIG. 3, supplied tovoltage-controlled oscillator 25 is converted by an analog-to-digitalconverter and recorded in a memory for each of 32 sampling positionsindicated at P₀, P₁, P₂ . . . P₃₀ and P₃₁ on FIG. 3, along that givenhorizontal line. In order to achieve the most accurate correction ofdeviations in the horizontal scanning rate, it would be desirable torecord correction values for each of such 32 sampling position alongeach of the over 200 horizontal lines within the effective picture orimage area 100 of a video field. Unfortunately, this would require thememory to have a very large storage capacity, which would result in anundesirable increase in cost.

In order to avoid the need for such a very large storage capacity in thememory, it is possible to provide apparatus which stores 32 correctionvalues, one for each of 32 horizontal sampling positions, P₀ -P₃₁, oneach of only 16 predetermined horizontal sampling lines, L₀, L₁, L₂. . .L₁₄, and L₁₅. These 16 sampling lines, L₀ -L₁₅, could be spaced at every16th horizontal line throughout a given video field so that, as shown inFIG. 3, 14 of the sampling lines would lie within the effective pictureportion 100 of a video field, and two lines, L₀ and L₁₅, would lie inportions of the raster scan occurring, respectively, before and afterthe effective picture portion 100, that is, above and below the imagearea. The storage of correction values for only one in each 16horizontal lines obviously greatly reduces the capacity required of thememory of such correction apparatus.

In the case in which correction values are stored in the memory for only16 sampling lines, L₀ -L₁₅, it is necessary to provide a way forsupplying a deflection correction signal to beam deflection device 16during the reproduction of the 15 horizontal lines which occur betweenadjacent sampling lines. The simplest way of doing this is by having theapparatus read from the memory the same 32 correction values during thescanning of each of those 15 horizontal lines that would be read frommemory at corresponding time periods during the scanning of theimmediately preceding sampling line. Although this technique is verysimple, the correction it makes in the horizontal scanning rate may beless than is desirable, as illustrated in FIG. 4.

The solid line 200 in FIG. 4 represents the changes in horizontalscanning rate that occur at a given horizontal location as a function ofchanges in vertical position within a video field. If these changes inhorizontal scanning rate are recorded only for the sampling linesoccurring at every 16th horizontal line, such as the sampling lines L₀-L₄ shown in FIG. 4, and if the correction values recorded for each ofthese sampling lines are used as the correction values for each of thesubsequent 15 horizontal lines, then the deflection correction signalssupplied to beam deflection device 16 will have a discontinuous functionas shown by broken line 202 in FIG. 4. The resulting discontinuities inthe deflection correction signals may give rise to the appearance ofhorizontal bands in the image produced by picture tube 10.

In accordance with the present invention, apparatus is provided whichstores correction values only for a small fraction of the total numberof horizontal lines in a video field, and which causes the horizontalscanning rate correction for those horizontal lines for which nocorrection values have been stored to vary continuously as a function ofvertical position, so as to produce an image upon screen 13 which issubstantially free of distortion. In the embodiment of the inventionillustrated by FIG. 1, such apparatus is shown to include a memory 40for storing correction values that represent deviations of thehorizontal scanning rate of the electron beam from a desired scanningrate at each of a plurality of predetermined horizontal samplingpositions along each of a plurality of predetermined sampling lines L₀-L₁₅ from among the horizontal lines scanned by the electron beam. Thecorrection apparatus also comprises a reading circuit including adigital-to-analog converter 50, a line address signal generating circuit51, a point address signal generating circuit 52 and an AND gate 53.Such reading circuit is capable of reading from memory 40 storedcorrection values corresponding to respective horizontal scanningpositions of the electron beam along a horizontal line being scanned bythe electron beam, so that each of the correction values being readrepresents a deviation of the horizontal scanning rate at a horizontalsampling position on one of the sampling lines for which correctionvalues have been recorded, with such horizontal sampling positioncorresponding to the actual horizontal scanning position of the electronbeam at the time of reading. The reading circuit is also capable ofproducing a sampling line correction signal in correspondence to each ofthe correction values read from memory 40. The apparatus provided inaccordance with this invention is also shown to include a scanned linecorrection signal generating circuit which, in FIG. 1, includes acoefficient signal generating circuit 60, a multiplying circuit 61, andan adding circuit 62. The circuit 62 is shown to include a capacitor 63,means constituted by transistors 64 and 65 for placing a first analogvoltage on capacitor 63, and means including transistors 66 and 67 andinverters 68 and 69 for placing a second analog voltage in series withcapacitor 63 once the latter has been charged. The scanned linecorrection signal generating circuit is capable of producing, for agiven horizontal scanning position of the electron beam, a scanned linecorrection signal of a value which is a function of the sampling linecorrection signal produced by the reading means in response to thatgiven horizontal scanning position and also a function of the verticalposition of the horizontal line being scanned. The scanned linecorrection signal produced, as aforesaid, is shown to be applied to beamdeflection device 16, so that a deviation of the horizontal scanningrate along the scanned line is substantially cancelled, by means of afilter having a capacitor 70 and a resistor 71, a differential amplifier72 connected to an adjustably fixed voltage source 73 and a wire 74.

The apparatus of FIG. 1 is further shown to be provided with a writingcircuit including a low-pass filter 80 and an analog-to-digitalconverter 81, for obtaining the correction values from the index signalprocessing circuit comprised of photodetector 20, bandpass filter 21 andphase-locked loop circuit 22, and for writing those correction values inmemory 40.

As will be described below, the horizontal scanning rate correctionapparatus disclosed in FIG. 1 is capable of producing scanned linecorrection signals during scanning of a horizontal line other than oneof the sampling lines for which correction values have been recorded bylinearly interpolating sampling line correction values produced by thereading circuit on the basis of the vertical position of the scannedline relative to the vertical positions of the sampling lines to whichsampling line correction signals correspond. As can be seen from thechain line 204 on FIG. 4, such linearly interpolated scanned linecorrection signals produce a very close approximation to the changes inthe horizontal scanning rate that actually occur at a given horizontallocation as a function of changes in the vertical position, asrepresented by solid line 200 on FIG. 4.

In the embodiment of the invention shown in FIG. 1, low-pass filter 80has its input connected to the output of low-pass filter 24 so as toreceive the control voltages supplied from low-pass filter 24 tovoltage-controlled oscillator 25. The output of low-pass filter 80 isconnected to the input of analog-to-digital converter 81, and themulti-bit digital output of analog-to-digital converter 81 is suppliedthrough a W contact of a mode switch 33b to a data input of memory 40,which may be a random-access memory capable of writing, storing andreading a plurality of multi-bit digital values each at a respectivememory address.

The line address signal generating circuit 51 receives vertical andhorizontal blanking signals V_(B) and H_(B), respectively, is alsoconnected through R contacts of mode switches 33c and 33d to receiveclock signals C₁ and C₂, respectively. Line address signal generatingcircuit 51 provides to memory 40 a line address signal of 4-bits whichconstitute the high order 4-bits of a 9-bit address word used to addresscorrection values in memory 40. These 4-bits indicate the sampling lineto which the correction value written or read in response to a givenaddress word will correspond. In response to the presence or absence ofinput from mode switches 33c and 33d, line address signal generatingcircuit 51 also provides a WRITE/READ signal to memory 40 to determinewhether the memory will write a value then being supplied to it at agiven memory address or read a correction value from such memory addressand supply it at its data output. Line address signal generating circuit51 also provides a sampling line gate signal to a W contact of a modeswitch 33e and to an input of coefficient signal generating circuit 60.Finally, circuit 51 provides a 4-bit relative vertical positionindicating signal to coefficient signal generating circuit 60.

Point address signal generating circuit 52 also receives the verticaland horizontal blanking pulses V_(B) and H_(B), respectively, and theoutput of AND gate 53 which has clock signal C₁ and the output from modeswitch 33e applied to its inputs. An R contact of switch 33e isconnected to a position potential and the W contact of switch 33ereceives the sampling line gate signal from circuit 51 as noted above.The output of point address signal generating circuit 52 is a 5-bitpoint address signal which is supplied to memory 40 to form the lower5-bits of the address word for that memory. These 5-bits are used toindicate to which of the 32 horizontal sampling positions on a givensampling line the correction values written or read from memory 40 willcorrespond. Point address signal generating circuit 52 also suitablyprovides a memory enable signal to memory 40 in response to pulses fromAND gate 53 which causes that memory to either write or read acorrection value, depending upon the value of the READ/WRITE signalsupplied to memory 40 by line address signal generating circuit 51.

The coefficient signal generating circuit 60 receives, as its inputs,the sampling line gate signal and the 4-bit relative vertical positionindicating signal, both from line address signal generating circuit 51,as described above. Coefficient signal generating circuit 60 alsoreceives, as an input, a complementing clock signal C_(X). Coefficientsignal generating circuit 60 provides a 4-bit coefficient signal as anoutput. This 4-bit coefficient signal is supplied to a multiplier inputof multiplying circuit 61.

The multi-bit correction values which are stored in memory 40 areadapted to be applied, through the R contact of mode switch 33b, to adigital input of digital-to-analog converter 50. The analog output ofconverter 50 is supplied to the multiplicand input of multiplyingcircuit 61. The output of multiplying circuit 61 is connected to theinput of adding circuit 62. As earlier noted, circuit 62 is comprised ofcapacitor 63 and the four transistors 64-67, which are desirablyfield-effect transistors, as shown. Capacitor 63 is connected in seriesbetween transistors 64 and 65, so that, when those transistors areturned ON, capacitor 63 is connected between the output of multiplyingcircuit 61 and ground. The gates of transistors 64 and 65 are connectedto the output of inverter 68 which receives clock signal C₂ as itsinput. Capacitor 63 is similarly connected in series between transistors66 and 67 so that, when those two transistors are turned ON, capacitor63 is connected between the output of multiplying circuit 61 and one ofthe two inputs of differential amplifier 72. The gates of transistors 66and 67 are connected to the output of inverter 69 which receives clocksignal C₁ as its input.

The input of differential amplifier 72 that receives the output fromadding circuit 62 is also connected to one end of parallel connectedcapacitor 70 and resistor 71 which, at their other end, are connected toground. The other input of differential amplifier 72 is connected toadjustably fixed voltage source 73.

The output of differential amplifier 72 is supplied through wire 74 to adeflection correction device 90 which constitutes part of beamdeflection device 16, and which may be desirably formed, as shown inFIG. 5, by a pair of ferrite cores 91 and 92 of semi-annular shape whichare placed in horizontally opposing positions around the neck portion ofpicture tube 10 near to the conventional fourth grid of the electrongun. A correction coil 93, having two ends 94 and 95 between which thedeflection correction signal is supplied, is wrapped around cores 91 and92 so that a magnetic field will be induced in the vertical direction,as indicated by the arrows on FIG. 5, to horizontally deflect theelectron beam.

In operation, the apparatus of FIG. 1 functions in one or the other oftwo modes: that is, a WRITE mode, in which correction values are derivedfrom the control voltage supplied to voltage-controlled oscillator 25and are recorded in memory 40; and a READ mode, in which correctionvalues are read from memory 40, and in which a scanned line correctionsignals are produced in accordance with the read correction values andare supplied to deflection correction device 90, so that deviations inthe horizontal scanning rate are substantially cancelled. The modeswitches 33a to 33e are preferably ganged so as to movable together forchange-over of the apparatus of FIG. 1 between the WRITE and the READmodes upon engagement of their movable contacts with the respective Wand R contacts. The mode switches 33a-33e may be operated automaticallyby a suitable mode control means (not shown) so that, whenever thetelevision receiver including the circuitry of FIG. 1 is initiallyturned on, all of the mode switches 33a-33e will connect theirrespective W contacts and the apparatus will be set to operate in theWRITE mode until all of the correction values have been written, andthen the mode control means will cause the apparatus to be changed-overto the READ mode in which the apparatus functions to reproduce videosignals on screen 13.

When mode switches 33a-33e are positioned to establish the WRITE mode,the input of drive circuit 34 is connected, through fixed contact W ofmode switch 33a, to an adjustably fixed voltage source 96 so that grid12 receives a suitable voltage to provide the electron beam projectedupon screen 13 with a constant beam current. In the WRITE mode, thisconstant intensity electron beam is made to scan across the screen 13under the influence of the horizontal and vertical beam deflectionsignals supplied to horizontal and vertical deflection coils 94, whichconstitute parts of beam deflection device 16 separate from deflectioncorrection device 90. As a result, an index signal is detected byphotodetector 20, filtered by bandpass filter 21 and supplied to PLLcircuit 22. This index signal has a frequency that varies in response toany changes in the scanning speed of the electron beam across screen 13under the influence of horizontal and vertical deflection coils 97.

In response to this index signal, PLL circuit 22 produces a controlvoltage at the output of low-pass filter 24 which varies in proportionto changes in the frequency of the index signal and, thus, in proportionto changes in the horizontal scanning rate. Such control voltage issupplied through low-pass filter 80 to the input of analog-to-digitalconverter 81, where it is converted to a multi-bit digital value whichis supplied through contact W of mode switch 33b to the input of memory40. The clock signal C₁ is supplied to the clocking input ofanalog-to-digital converter 81 so that it converts the control voltageto a digital value at each pulse of the clock signal C₁ as shown in FIG.6. As is indicated in FIG. 6, each of the clock pulses of the signal C₁corresponds to one of the horizontal sampling positions P₀ -P₃₁ forwhich correction values are to be stored in memory 40.

In the WRITE mode, mode switches 33c and 33d prevent clock signals C₁and C₂ from being supplied to line address signal generating circuit 51and, as a result, the WRITE/READ signal supplied by circuit 51 to memory40 is set so as to cause memory 40 to operate in the WRITE mode.Further, in the WRITE mode, line address signal generating circuit 51counts horizontal blanking pulses H_(B) and is reset by verticalblanking pulses V_(B), so as to keep within circuit 51 a count of thenumber of the horizontal line within a video field currently beingscanned by the electron beam of picture tube 10. The 4 high-order bitsof this count are supplied to memory 40 as the line address signal andthe 4 low-order bits of this count are supplied to coefficient signalgenerating circuit 60 as the relative vertical position indicatingsignal. When the 4 low-order bits of this count are all equal to zero,the electron beam of picture tube 10 is sweeping one of the samplinglines L₀ -L₁₅ and the sampling line gate signal supplied from circuit 51to contact W of switch 33e goes high. The relative timing of these 16sampling lines relative to the vertical field interval is illustrated inFIG. 7.

In the WRITE mode, when each of these sampling lines L₀ -L₁₅ is scannedby the electron beam, the high value of the sampling line gate signalcauses clock signal C₁ to be gated through AND gate 53 to point addresssignal generating circuit 52 which counts the pulses of clock signal C₁occurring at each of the 32 horizontal sampling positions P₀ -P₃₁ oneach sampling line. The 5-bit count in point address signal generatingcircuit 52 is reset by the vertical and horizontal blanking signalsV_(B) and V_(H), respectively. Thus, it can be seen that memory 40 issupplied with a 9-bit address word, the lower 5 bits of which areincremented by one each time a horizontal sampling position P₀ -P₃₁ isscanned, and the upper 4 bits of which are incremented by one each timea new one of the sampling lines L₀ -L₁₅ is ready to be scanned.

In addition, point address signal generating circuit 52 provides amemory enable signal to memory 40 in response to each pulse in clocksignal C₁ so that, in the WRITE mode, at each of the 32 horizontalsampling positions P₀ -P₃₁ of each sampling line L₀ -L₁₅, a correctionvalue corresponding to the control voltage supplied tovoltage-controlled oscillator 25 during the scanning of the respectivehorizontal sampling position will be recorded in memory 40 at an addressuniquely identifying the horizontal sampling position and sampling linefor which it was recorded.

The function of low-pass filter 80 is to remove from the control voltagesupplied to analog-to-digital converter 81 those variations that have ahigh frequency relative to the frequency at which converter 81 samplesthe control voltage and converts it into digital values, so that eachresulting correction value stored in memory 40 represents the averagevalue of the control voltage at the time that each such sampling ismade.

In the embodiment of the invention shown in FIG. 1, memory 40 is arandom-access memory (RAM). Random-access memory normally loses thevalues stored in it when its power is shut off. For this reason, theapparatus of FIG. 1 should be set to its WRITE mode briefly each time itis turned ON, so that suitable correction values will be stored inmemory 40 before the subsequent operation of the apparatus in the READmode.

After a complete set of correction values has been stored in memory 40,the apparatus of FIG. 1 can be changed-over to the READ mode in whichall of the mode switches 33a-33e engage their respective R contacts. Inthe READ mode, switch 33a connects the input of drive circuit 34 throughits contact R to the output of gate circuit 32. Thus, in the READ mode,color signals E_(R), E_(G) and E_(B) are sequentially supplied throughdrive circuit 34 to grid 12 in the manner described above, so that colorvideo images are reproduced upon screen 13. Further, in the READ mode,scanned line correction signals are generated and supplied to correctiondevice 90 so as to substantially cancel deviations in the horizontalscanning speed.

In the READ mode, vertical and horizontal blanking pulses V_(B) andV_(H) are applied to line address signal generating circuit 51, andswitches 33c and 33d are positioned to supply clock signal C₁ and clocksignal C₂, which is shifted in phase relative to clock C₁ as shown inFIG. 8, to line address signal generating circuit 51.

In response to the receipt of those clock signals C₁ and C₂ line addresssignal generating circuit 51 suitably causes the WRITE/READ signalsupplied to memory 40 to condition such memory in its READ mode. At therising edge of each clock signal C₁ the 4-bit line address signalsupplied by the line address generator 51 to memory 40 is set to thesame value as the 4 most significant bits contained within the counterof line address signal generating circuit 51. At the rising edge ofclock signal C₂ the 4-bit line address signal will be set to a value onegreater than the 4 most significant bits of the horizontal line countthen contained within line address signal generating circuit 51. Forexample, when the line currently being scanned on screen 13 ranges fromthe first sampling line L₀ to the horizontal line occurring immediatelybefore the next sampling line L₁, the line address signal will be set to"0000" at the rising edge of clock signal C₁ and will be set to "0001"at the rising edge of clock signal C₂ as shown in FIG. 8.

In the READ mode, mode switch 33e causes a high signal to be supplied toAND gate 53 during the scanning of all horizontal lines. As a result,point address signal generating circuit 51 counts a clock pulse andproduces a memory enable signal at each of the 32 horizontal samplingpositions P₀ -P₃₁ during each of the horizontal lines of the videofield. As a result of counting clock pulses C₁, point address signalgenerating circuit 51 provides a 5-bit point address signal to memory 40which corresponds to the number of the horizontal sampling positions P₀-P₃₁ currently being scanned.

Thus, as is indicated in FIG. 8, two address values are supplied tomemory 40 at each horizontal sampling position P₀ -P₃₁ scanned by theelectron beam on screen 13, a first address value being the address ofthe corresponding horizontal sampling position on the highest numbersampling line which the electron beam has started to scan within thecurrent video field, and the second supplied address value being theaddress of the corresponding horizontal sampling position on the nextsampling line to be scanned by the electron beam. As a result, for eachhorizontal sampling position scanned by the electron beam, twocorrection values are read from memory 40, namely, a first correctionvalue U corresponding to an upper sampling line, which is the samplingline at or immediately above the horizontal line currently beingscanned, and a second correction value D corresponding to a lowersampling line, which is the sampling line immediately below thehorizontal line currently being scanned. In the READ mode, thesemulti-bit digital correction values U and D are applied through contactR of mode switch 33b to the digital input of digital-to-analog converter50 where they are continuously converted into a corresponding analogvoltage supplied to the multiplicand input of multiplying circuit 61.

As mentioned above, the line address signal generating circuit 51supplies a sampling line gate signal and a 4-bit relative verticalposition signal to coefficient signal generating circuit 60. Coefficientsignal generating circuit 60 also receives a complementing clock signalC_(X), which rises at the rising edge of clock signal C₂ and falls offat the rising edge of the clock signal C₁, as shown in FIG. 8.Coefficient signal generating circuit 60 produces a 4-bit weightingcoefficient W_(U) when the complementing pulse C_(X) is "0" or lowduring the time that the upper sampling line correction value U is readfrom memory 40, and circuit 60 produces a weighting coefficient W_(D)when the complementing signal C_(X) is "1" or high during the time thatthe lower sampling correction value D is read from memory 40. These4-bit weighting coefficients are each capable of representing the values0, 1/16, 2/16, . . . 15/16 and 16/16. For each horizontal line, scannedthe weighting coefficient W_(D) equals 1/16 times the value of the lower4-bits of the horizontal line count contained within line address signalgenerating circuit 51, and the weighting coefficient W_(U) equals1-W_(D). Thus, for example, as is shown in FIG. 8, upon the reproductionof sampling line L₀, W_(U) is 16/16 and W_(D) is 0. Similarly, duringthe reproduction of the next horizontal line, that is, the line L₀ +1,W_(U) is 15/16 and W_(D) is 1/16. And finally upon reproduction of thelast horizontal line before the next sampling line L₁, that is, upon thereproduction of the horizontal line L₁ -1, W_(U) is 1/16 and W_(D) is15/16. As can be seen from this example, the coefficients W_(U) andW_(D) change linearly in accordance with the vertical position of thehorizontal line to be reproduced.

Clock signals C_(X) cause coefficient W_(U) to be applied to multiplyingcircuit 61 at the same time that the upper sampling line correctionvalue U is being read from memory 40 and converted into an analogvoltage by digital-to-analog converter 50. In a similar manner, theclock signals C_(X) cause the coefficient W_(D) to be supplied tomultiplying circuit 61 at the same time that the lower sampling linecorrection value D is being read from memory 40 and converted into ananalog voltage by digital-to-analog converter 50. As a result,multiplying circuit 61 first has an output voltage corresponding toW_(U) ·U and then has an output voltage corresponding to W_(D) ·D foreach of the horizontal sampling positions P₀ -P₃₁ scanned by theelectron beam on screen 13. These alternate weighted line samplingsignals W_(U) ·U and W_(D) ·D are supplied to the input of addingcircuit 62.

During the time that signal W_(U) ·U is applied to adding circuit 62,clock signal C₂ has a brief low voltage. This brief low voltage isinverted by inverter 68 to produce a brief positive pulse which issupplied to the gates of transistors 64 and 65 so as to turn thosetransistors ON and to apply the voltage of signal W_(U) ·U acrosscapacitor 63. This charge remains on capacitor 63 upon termination ofthe brief positive pulse from inverter 68.

The output of multiplying circuit 61 is such that it can either addcharge to or remove charge from capacitor 63 so as to maintain its ownoutput voltage at the voltage W_(U) ·U. As a result, the voltage left oncapacitor 63 at the end of the time that transistors 64 and 65 areturned ON is independent of the charge that was on capacitor 63 prior tothat time. During the time that the signal W_(D) ·D is supplied to theinput of adding circuit 62, clock signal C₁ has a brief low pulse. Thislow pulse is inverted by inverter 60 so as to produce a brief positivepulse which is supplied to the gates of transistors 66 and 67 so as toturn ON those transistors and thereby apply the voltage upon capacitor63 in series with the output of multiplying circuit 61 to one of theinputs of differential amplifier 72. Thus, it can be seen that thesignal supplied to that one input of differential amplifier 72 is equalto the sum of W_(U) ·U and W_(D) ·D.

It will be noted that, in FIG. 8, the first negative pulse in clocksignal C₁ is not shown to occur until near the end of the period P₀.Such first negative pulse in the clock signal C₁ corresponds with thetime for which a correction value was recorded or stored in the memoryfor the horizontal sampling position P₀. This is desirable because it isnot until the end of the period P₀ shown in FIG. 8 that the scanned linecorrection signal W_(U) ·U+W_(D) ·D is applied through differentialamplifier 72 to the correction coil 93 of device 90. Thus, it can beseen that a scanned line correction signal calculated from correctionvalues recorded at the time that the horizontal sampling position P₀ wasscanned is supplied to the correction coil 93 at the time that acorresponding horizontal sampling position P₀ is being scanned duringthe reproduction of a video signal. It will be noted that the upper linecorrection value U is read from memory 40 during the first half of thetime period P₀ indicated on FIG. 8 because clock signal C₁ was the lastsignal to have a rising edge during the previous horizontal interval.

The function of the filter comprised of capacitor 70 and resistor 71 isto hold the scanned line correction signal supplied by adding circuit 62for one horizontal sampling position until the next scanned linecorrection signal is supplied from adding circuit 62 in correspondencewith the next horizontal sampling position.

The apparatus shown in FIG. 1 is designed so that the scanned linecorrection signals supplied to coil 93 generate a magnetic field which,at any time, is sufficient to substantially cancel unwanted deviationsin the horizontal scanning rate. As discussed above, the correctionvalues stored in memory 40 vary in proportion to the frequency of theindex signal at various horizontal sampling positions P₀ -P₃₁ along eachof the horizontal sampling lines L₀ -L₁₅ when the apparatus was lastoperated in the WRITE mode. When the apparatus is operated in the READmode, the correction values corresponding to the horizontal samplingposition currently being scanned on the two nearest sampling lines areread from memory 40 and converted into proportionally correspondinganalog voltages by digital-to-analog converter 50. Then by means ofmultiplying circuit 61 and adding circuit 62 a scanned line correctionsignal is produced by linearly interpolating between the analog voltagescorresponding to the two correction values read from memory 40 on thebasis of the relative distance of the horizontal line being scanned fromeach of the sampling lines for which each of those correction values wasrecorded. As a result, the scanned line correction signal supplied tothe input of differential amplifier 72 corresponds fairly accruately tothe horizontal scanning rate on each of the horizontal lines scanned onscreen 13, as is indicated by the close aproximation of chain line 204of FIG. 4 to the solid line 200.

The voltage applied to the second input of differential amplifier 72from adjustably fixed voltage source 73 is selected to equal the outputof adding circuit 62 that would result upon the reproduction of asampling line L₀ -L₁₅ when the upper sampling line correction value Uread from memory 40 represented the desired horizontal scanning rate. Inother words, the voltage suppied to the second input of amplifier 72 isone which corresponds with the desired scanning rate. Thus, the voltageof the scanned line correction signal produced at the output ofdifferential amplifier 72 varies in proportion to a determinedhorizontal scanning rate error, that is, in proportion to the differencebetween a desired horizontal scanning rate and substantially thehorizontal scanning rate which the electron beam would have without anycorrection.

The voltage of the scanned line correction signal is integrated by theinductance of coil 93 to produce a current in that coil and a resultingmagnetic field which are both proportional to the integral of thecalculated horizontal scanning rate error. The horizontal scanning rateof the electron beam is altered by an amount substantially proportionalto the rate of change of the magnetic field created by the flow of thecurrent in coil 93. As a result, the horizontal scanning rate is alteredby an amount substantially proportional to the differential of theintegral of the horizontal scanning rate error, that is, by an amountsubstantially proportional to the horizontal scanning rate error itself.Thus, by applying the deflection correction signal to coil 93 with theproper polarity, it is possible to substantially cancel the horizontalscanning rate error of picture tube 10.

By way of summary, it will be appreciated, that in the above-describedapparatus in accordance with this invention, the horizontal scanningrate is corrected for each horizontal line of the video field byreference to correction values representing deviations of the horizontalscanning rate of the electron beam and which are recorded or stored foronly a small percentage of the total number of horizontal lines. As aresult, the memory capacity required in this apparatus is greatlyreduced, even though it is possible for the correction of the horizontalscanning rate for those lines for which no correction values wererecorded to correspond quite closely to the correction that would bemade had correction values been recorded for each individual horizontalline.

The apparatus disclosed in FIG. 1 not only compensates for deviations inthe horizontal scanning rate which can be predicted from the design ofthe cathode-ray tube, but it also compensates for deviations in thehorizontal scanning rate which may result from the unpredictableirregularity of circuit components within a given individual cathode-raytube or associated therewith. As a result, the horizontal scanning rateof such cathode-ray tubes can be kept substantially constant, preventingdistortion of the shapes of the images reproduced and preventing colormisregistration.

The substantially constant frequency of the index signal made possibleby this invention enables PLL circuit 22 to maintain synchronism withthe index signal, even when that signal is weak, for example, due to alow beam current associated with the reproduction of a dark image areaupon the screen of the picture tube. As a result, this invention makesit possible to lower the minimum electron beam current while stillmaintaining synchronization of the PLL circuit, allowing low luminanceportions of a video signal to be darker when reproduced and, thus,improving the contrast of the resulting displayed image.

As discussed above, the apparatus shown in FIG. 1 has a WRITE mode inwhich new correction values can easily be recorded in memory 40, forexample, when the apparatus is first turned ON. It will be apparent thatan embodiment of the present invention could be provided which isidentical to that of FIG. 1 except for the fact that it does not providefor operation in a WRITE mode. In such an alternative embodiment of theinvention, memory 40 would be comprised of a read-only memory (ROM) andcorrection values would be written in memory 40 by means of circuitryexternal to the apparatus, for example, circuitry at the factory wherethe apparatus is manufactured, or at a repair shop where such apparatusmight be serviced. Although this alternative embodiment would have thedisadvantage of not being able to have its correction values renewed asfrequently, or as easily, as the apparatus shown in FIG. 1, it wouldhave the offsetting advantage of avoiding the need for the devices inthe apparatus of FIG. 1, such as, mode switches 33a-33e, low-pass filter80, and analog-to-digital converter 81, required for change-over to theWRITE mode.

It will also be apparent that, in a horizontal scanning rate correctionapparatus according to the present invention, after an upper samplingline correction value U and a lower sampling line correction value Dhave been read from memory 40 in digital form, the could each bemultiplied by their respective weighting coefficient, W_(U) and W_(D),and the resulting products could each be added digitally so as toproduce a digital value W_(U) ·U+W_(D) ·D which could then be convertedto an analog voltage and supplied to one of the inputs of differentialamplifier 72.

In addition, it will be apparent that it would be possible to provide anembodiment of the present invention which used two memory devices eachcontaining all of the correction values recorded for each of thesampling lines. This would make it possible for the apparatus tosimultaneously read the upper sampling line correction value U from onememory and the lower sampling line correction value D from the othermemory.

It will also be apparent that, with appropriate changes in the meanswhich delivers the scanned line correction signal to the beam deflectiondevice, the beam deflection device which is used to correct for errorsin the horizontal scanning rate could be of an electrostatic type.Furthermore, it will be apparent that the desired deflection correctioncould be effected through the main horizontal deflection coil 97 of thecathode-ray tube. For example, a saturable reactor could be employedwhich has the deflection correction signal supplied to its primarywinding and which has its secondary winding connected in series with thehorizontal deflection winding of the cathode-ray tube so that thedeflection correction signal can be used to vary the magnitude of thehorizontal beam deflection signal which would normally be applied to thehorizontal deflection winding.

Furthermore, it will be apparent that the horizontal scanning ratecorrection apparatus according to this invention can be used withcathode-ray tubes in video apparatus other than television receivers,such as, for example, in computer terminals.

Having described specific preferred embodiments of the invention withreference to the accompanying drawings, it is to be understood that theinvention is not limited to those precise embodiments, and that variouschanges and modifications may be effected therein by one skilled in theart without departing from the scope or spirit of the invention asdefined in the appended claims.

We claim:
 1. Horizontal scanning rate correction apparatus for acathode-ray tube having a screen, means for projecting an electron beamupon said screen, and a beam deflection device supplied with at leasthorizontal and vertical beam deflection signals for causing said beam torepeatedly scan across said screen in a vertical succession ofhorizontal lines, said apparatus comprising:memory means for storing aplurality of correction values representing deviations of the horizontalscanning rate of said beam from a desired scanning rate at a pluralityof predetermined horizontal sampling positions along each of a pluralityof predetermined sampling lines from among said horizontal lines;reading means for reading a selected one of said stored correctionvalues for each horizontal scanning position of said electron beam alonga scanned one of said horizontal lines, each correction value that isread representing said deviation at a corresponding one of saidhorizontal sampling positions on one of said sampling lines, saidreading means including means for producing a sampling line correctionsignal corresponding to each of said read correction values; means forproducing, for each said horizontal scanning position, a scanned linecorrecion signal which is an interpolated function of the sampling linecorrection signal of the last-scanned sampling line, the correspondingsampling line correction of the next-to-be-scanned sampling line, andthe vertical position of the respective scanned line relative to saidlast-scanned and said next-to-be-scanned sampling lines; and means forsupplying said scanned line correction signal to said beam deflectiondevice so that said deviation of the horizontal scanning rate along saidscanned line is substantially cancelled.
 2. Horizontal scanning ratecorrection apparatus for a cathode-ray tube having a screen, means forprojecting an electron beam upon said screen, and a beam deflectiondevice supplied with at least horizontal and vertical beam deflectionsignals for causing said beam to repeatedly scan across said screen in avertical succession of horizontal lines, said apparatuscomprising:memory means for storing a plurality of correction valuesrepresenting deviations of the horizontal scanning rate of said beamfrom a desired scanning rate at a plurality of predetermined horizontalsampling positions along each of a plurality of predetermined samplinglines from among said horizontal lines, in which the number of saidsampling lines is substantially less than the number of said horizontallines; reading means for reading a selected one of said storedcorrection values for each horizontal scanning position of said electronbeam along a scanned one of said horizontal lines, each correction valuethat is read representing said deviation at a corresponding one of saidhorizontal sampling positions on one of said sampling lines, saidreading means including means for producing a sampling line correctionsignal corresponding to each of said read correction values; means forproducing, for each said horizontal scanning position, a scanned linecorrection signal which is a function of said sampling line correctionsignal and the vertical position of the respective scanned line, saidmeans for producing the scanned line correction signal being operativeto adjust said sampling line correction signal in correspondence to thedeparture of said scanned line from said sampling line to which saidread correction values correspond; and means for supplying said scannedline correction signal to said beam deflection device so that saiddeviation of the horizontal scanning rate along said scanned line issubstantially cancelled.
 3. Horizontal scanning rate correctionapparatus according to claim 2; in which said means for producing thescanned line correction signal includes interpolating means forproducing said scanned line correction signal by interpolating betweensampling line correction signals on the basis of the vertical positionof said scanned line relative to the vertical positions of the samplinglines to which said read correction values correspond.
 4. Horizontalscanning rate correction apparatus according to claim 3; in which saidsampling lines are separated from each other by unsampled horizontallines; and in which, when said scanned line is located between twosuccessive ones of said separated sampling lines, two of said correctionvalues are read which respectively correspond to said two successivesampling lines.
 5. Horizontal scanning rate correction apparatusaccording to claim 4; in which said sampling lines are separated fromeach other by 15 of said unsampled lines.
 6. Horizontal scanning ratecorrection apparatus according to claim 4; in which said beam deflectiondevice includes horizontal and vertical deflection coils for receivingsaid horizontal and said vertical beam deflection signals, respectively,and a separate correction coil placed on a yoke separate from saidhorizontal and vertical deflection coils for receiving said scanned linecorrection signal.
 7. Horizontal scanning rate correction apparatusaccording to claim 4; in which said means for producing the scanned linecorrection signal includes multiplying means producing two weightedsampling line correction signals by multiplying the sampling linecorrection signals corresponding to said two read correction values byan interpolation coefficient which varies in inverse proportion to thevertical distance of said scanned line from the sampling line to whichthe respective read correction value corresponds, and means for addingtogether said two weighted sampling line correction signals to producesaid scanned line correction signal.
 8. Horizontal scanning ratecorrection apparatus according to claim 7; in which said weightedsampling line correction signals are supplied to said means for addingin the form of respective analog voltages; and in which said means foradding includes a capacitor, means for placing a first of said analogvoltages on said capacitor, and means for then placing a second of saidanalog voltages in series with said charged capacitor to produce avoltage which corresponds to the sum of said analog voltages for use assaid scanned line correction signal.
 9. Horizontal scanning ratecorrection apparatus according to claim 4; in which said reading meansincludes line address signal generating means for determining thevertical position, within each video field of said scanned line and forgenerating, as a function of said vertical position, line addresssignals effective in said memory means for controlling which twosuccessive ones of said sampling lines are to have their correspondingcorrection values read, and point address signal generating means forgenerating a point address signal corresponding to said horizontalscanning position and which is effective in said memory means forcontrolling which of said horizontal sampling positions along each ofsaid two successive sampling lines addressed by said line address signalgenerating means is to have its corresponding correction value read. 10.Horizontal scanning rate correction apparatus according to claim 9; inwhich, for each point address signal supplied by said point addresssignal generating means, said reading means sequentially reads thecorrection value corresponding to a first and then to a second of saidtwo successive sampling lines addressed by said line address signalgenerating means.
 11. Horizontal scanning rate correction apparatusaccording to claim 9; in which said memory means has two simultaneouslyaddressable portions, each containing said correction values for arespective one of said sampling lines; and in which, for each pointaddress signal supplied by said point address signal generating means,said reading means simultaneously reads from said two portions thecorrection values corresponding to said horizontal scanning position insaid two successive sampling lines addressed by said line address signalgenerating means.
 12. Horizontal scanning rate correction apparatusaccording to claim 4; in which said cathode-ray tube is an index typecolor cathode-ray tube having a plurality of index elements positionedto be struck by said electron beam as it scans across said screen, colorswitching circuitry for determining which of a plurality of colorsignals modulates the intensity of said beam, and an index signalprocessing circuit for producing an index signal of a frequencydetermined by the frequency of the incidence of said beam upon saidindex elements as it scans across said horizontal lines, and forcontrolling said color switching circuitry; and in which means areprovided for producing said correction values from said index signalprocessing circuit when said sampling lines are scanned by the beam inaccordance with said horizontal and vertical beam deflection signals.13. Horizontal scanning rate correction apparatus according to claim 12;in which said index elements are spaced across said horizontal lines sothat the frequency of said index signal varies in proportion tovariations in said horizontal scanning rate, and in which saidcorrection values, as stored in said memory means, record changes in thefrequency of said index signal.
 14. Horizontal scanning rate correctionapparatus according to claim 13; in which said index signal processingcircuit includes a phase-locked loop which has a phase comparatorreceiving said index signal as a first input signal, avoltage-controlled oscillator, means connected to the output of saidphase comparator for supplying a control voltage to saidvoltage-controlled oscillator, and means for supplying a second inputsignal to said phase comparator the frequency of which is controlled bythe output of said voltage-controlled oscillator so that said output ofthe phase comparator varies with changes in said index signal to providea corresponding change in the output of said voltage-controlledoscillator; and in which said correction values are derived from saidcontrol voltage supplied to said voltage-controlled oscillator. 15.Horizontal scanning rate correction apparatus according to claim 12; inwhich said memory means includes a read-only memory for storing saidcorrection values in digital form; and in which said correction valuesproduced from said index signal processing circuit have been written insaid read-only memory by circuitry external to said horizontal scanningrate correction apparatus.
 16. Horizontal scanning rate correctionapparatus according to claim 12; further including writing means forwriting said correction values in said memory means, and mode switchingmeans for switching said apparatus between a READ mode in which saidreading means, said means for producing the scanned line correctionsignal, and said means for supplying said scanned line correction signalto said beam deflection device are operative, and a WRITE mode in whichsaid writing means is operative.
 17. Horizontal scanning rate correctionapparatus according to claim 16; in which said cathode-ray tube has anelectrode to which a signal is supplied for controlling the intensity ofsaid electron beam; and in which said apparatus further comprises meansfor supplying a signal to said electrode when said mode switching meansswitches said apparatus to said WRITE mode so that said beam has asubstantially constant current as it scans across said index elementsduring said WRITE mode.
 18. Horizontal scanning rate correctionapparatus according to claim 4; in which said memory means includes aread-only memory for storing said correction values in digital form.